Batch Script Clarification

[GreenPhenz]

I've seen the post on how to use this sricpt for batches:
for i in {1..10}; do ffs reference/$i.mkv -i in/$i.srt -o out/$i-synced.srt done
But I don't understand where the input files are supposed to go. What is the reference/, in/, and out/ supposed to be replaced with? Also, is the output file supposed to already be created or can the program automatically make one (and if so how can I direct which folder for it to go in?)?

Also, I'm on mac if that changes anything